a) Field of the Invention
This invention relates to a surface light source device to be used as a backlight for a liquid crystal display unit.
b) Description of the Prior Art
FIG. 1 shows a conventional surface light source device that has a light-conducting member. This device comprises a linear light source 1, such as a cold-cathode fluorescent tube, a light-conducting member 2 that has an edge surface of incidence 2a disposed adjacent the light source 1, a diffusion plate 3 that is located on or adjacent a front surface 2b (a top surface in FIG. 1) of the light-conducting member 2, and a reflective surface that is disposed adjacent a rear surface 2c (a lower surface in FIG. 1) of the light conducting member 2. In this surface light source device, an incident light from the source 1 is directed toward the edge surface of incidence 2a of the light-conducting member 2. The light-conducting member 2 conducts the light toward a rearward edge surface 2d by total internal reflection from the surfaces 2b and 2c.
In the conventional device, however, the surface 2c of the light-conducting member 2 is provided with either a roughened surface or spots 2e formed by painting to scatter part of the conducted light, in such a way that part of the light, while travelling toward the surface 2d, spreads out from the surface 2b without being reflected by the surface. Thus, in the surface light source device of FIG. 1, when the rear surface 2c of the light-conducting member 2 has a rough surface or forms an array of spots 2e, as shown in FIG. 1, so that, while the incident light from the source 1 is advanced passing through the light-conducting member 2, part of this light spreads out from the surface 2b and then passes through the diffusion plate 3 to form a diffused light, which is available for use as a surface light source.
In this surface light source device, it is necessary to ensure that the diffused light that has passed through the diffusion plate 3 should appear uniformly over an entire surface of the diffusion plate 3.
Also, because this surface light source device can be used, for example, as a back-light of a liquid crystal display, this requires the diffused light to be bright enough for such an application. Thus, the surface light source device is provided with means to increase the brightness.
Typical surface light source devices of the prior art that have means to increase the brightness, are disclosed in Japanese Patent Kokai Publication No. 3-189679A and Japanese Utility Kokai Publication No. 3-31782A. Like the device of FIG. 1, each of these devices comprises a light source, a light-conducting member, a diffusion plate and a reflective plate. The surface 2c of the light-conducting member 2, which is opposite to the front or emitting surface 2b, i.e. the surface 2c adjacent to the reflective surface 4, forms a large number of minute pyramids which are arranged concavely or convexly to increase the brightness on the diffusion plate. Various sizes of these pyramids are arranged according to the area chosen to produce a uniform brightness distribution.
These prior art devices, however, have a problem in that, although it is possible to increase the brightness if the pyramid has a relatively large surface area, it is difficult to produce a uniform brightness distribution on the diffusion plate 3. Also, the pyramid formed on the light-conducting member has a disadvantage in that the light will shine at their corners.